1. Field of the Invention
This invention relates to a transducer for generating an electrical signal proportional to the difference in oxygen partial pressure between gases disposed on a pair of its surfaces and to support configurations for this and other sensors.
2. Prior Art
Hickam U.S. Pat. No. 3,347,767 discloses a class of sensors employing the galvanic effect to generate an electrical signal proportional to the difference between the oxygen partial pressure of two oxygen containing gases, one of which has a known oxygen partial pressure and acts as a reference. The gases are separated by an oxygen-ion conductive solid electrolyte such as yttria stabilized zirconia. The side of the electrolyte wall contacted by the unknown gas is coated with a porous platinum catalyst. Electrodes are attached to both sides of the electrolyte wall. Assuming that the known gas is air and the unknown gas is rich in oxygen, a concentration cell is formed between the oxygen in the air and the oxygen in the gas. The output voltage of this concentration cell is a function of the oxygen concentration in the unknown gas. If the unknown gas is the exhaust product from a combustion process and is deficient in oxygen, the unburned combustibles in the exhaust act as fuel and the oxygen coming through the electrolyte through the air acts as an anti-fuel to form a fuel cell. The output voltage produced in this case indicates whether the exhaust gas is rich in combustion products or oxygen.
When used in conjunction with an appropriate temperature sensing and/or control device, such galvanic cell transducers are particularly useful in sensing the oxygen content of internal combusion engine exhaust for the purpose of controlling the input air-fuel ratio to obtain optimum combustion conditions.
One problem associated with the operation of these sensors involves the loosening and peeling of the platinum catalyst layer as a result of water formation between the platinum and the oxygen-ion conductive electrolyte. This water is formed when the oxygen ions passing through the electrolyte from the reference gas meets with hydrogen that diffuses through the platinum at the platinum-electrolyte interface, since platinum as well as palladium, the other commonly employed platinum catalyst are both pervious to hydrogen. The catalyst causes the hydrogen and oxygen to react to form gaseous products and some water. The scrubbing effect of the flowing unknown gas on the platinum layer tends to displace it where it has been loosened by the formation of water on its undersurface and the platinum peels away.
Another difficulty is encountered in the use of these galvanic cell sensors, as well of other gas sensors, to measure the oxygen content of a combustion gas. Prior art galvanic cells take the form of tubes through which the unknown gas is passed, with a known oxygen-containing gas surrounding the outer portion of the tube. Usually, the tube forms part of a by-pass which taps off a small portion of the gas flowing through the vehicle exhaust system and then reintroduces the gas to the mainstream after passing it through the sensor. The cost of providing the diverting tubing may equal or exceed the initial cost of the sensor in such an arrangement.
3. Summary of the Invention
The present invention relates to an improved form of galvanic cell oxygen sensor employing a platinum family catalyst which simply and inexpensively eliminates the tendency of the platinum to peel away as a result of water formation, and additionally to a sensor package which allows such a galvanic sensor, or other forms of combustion-gas analyzers, to be inserted directly into the exhaust stream through a wall of the exhaust conduct, without the need for any diverting tubing.
Rather than employing a porous catalytic layer applied directly to one surface of the oxygen-ion conductive solid electrolyte, as was done in accordance with the prior art, in sensors formed in accordance with the present invention the surface of the electrolyte to which the catalyst is to be applied is first coated with a porous layer of solid electronic conductor material, preferably a transition metal oxide, which is impermeable to hydrogen molecules and oxygen ions. This layer may be applied by a variety of techniques including vapor deposition.
The platinum family catalyst is then deposited over the electrolyte tube and the conductor layer in a random manner so that at some points the surface of the electrolyte is coated with the conductor and the catalyst on top of the conductor; at other points the surface is only covered by the conductor; at still other points the catalyst is directly in contact with the electrolyte surface, having been deposited in the voids in the conductor coating; and at points the electrolyte is uncoated so as to be directly in contact with the gas flowing past its surface. Since the conductor is impervious to both hydrogen molecules and oxygen ions, no reaction takes place at the interface between the conductor and the catalyst and the catalyst remains firmly adhered to the conductor.
At those points where the catalyst is directly in contact with the electrolyte surface water may be formed by the reaction between the oxygen ions diffusing through the electrolyte and the hydrogen molecules diffusing through the catalyst from the unknown gas. The peeling that results from this water formation will tend to remove the platinum from direct contact with the electrolyte surface so that after use for some time only the catalyst which is plated over the conductor will remain in place. The edges of these catalyst coatings over the conductor will be in sufficient proximity to the electrolyte to produce a reaction between the exhaust gas products and the oxygen ions diffusing through the electrolyte.
While this technique may be used to prevent the catalyst peeling on conventional galvanic sensors taking the form of tubes through which the unknown gas is passed, the present invention provides an alternative sensor configuration which is useful in connection with other forms of gas sensors as well as the galvanic type previously described. In the preferred embodiment of the invention, which will subsequently be disclosed in detail, a small diameter zirconia tube is closed at one end and coated first with the porous transition metal oxide conductor and then with the porous platinum family catalyst. Electrodes are attached to the inner and outer sides of this tube and the tube is encased in a ceramic sleeve so that only the closed end of the tube projects from the sleeve. A metal collar having threads formed on its outer surface surrounds the ceramic collar. The collar is hollow so that the interior of the zirconia tube is exposed to the atmosphere which acts as a reference source of oxygen. The two electrodes are joined to conductors formed on a printed circuit board which has a connector end projecting out from the rear of the ceramic sleeve. To accommodate the transducer, a threaded hole is formed in a side wall of the exhaust conduit from the combustion chamber and the threaded sleeve on the transducer is screwed into this opening so that the closed end of the zirconia tube projects into the exhaust conduit. The connectors project outwardly from the exterior of the conduit and the open end of the tube is exposed to the atmosphere.
Not only does this arrangement greatly simplify the "plumbing" necessary to connect a gas sensor to an exhaust stream, but it also allows for easy inspection and replacement of the sensor.
This transducer support arrangement may be employed with other forms of transducers employed with gas conduits as well as the galvanic cell type. For example, the transition metal oxides exhibit a resistance proportional to their ambient oxygen partial pressure and this effect has been utilized to measure the oxygen content of exhaust gases. In an embodiment of the invention subsequently described in detail, a sensor of this transition metal oxide type is supported in a threaded coupler of the same type used with the galvanic cell transducer. The transition metal oxide sensor employs a unique hermetically sealed heating element formed by depositing the resistance element on a ceramic substrate and then fusing that substrate to the back of a second ceramic section which has a metal oxide sensor and conductors formed on its front side. A pair of conductors joining the deposited heating element to the connector end are also fused between two substrates.
Using similar techniques, heating and/or cooling elements could be formed for other sensors such as the galvanic cell type.